The invention relates to a white, biaxially oriented polyester film having at least one base layer comprising a thermoplastic polymer and at least one acrylate-containing layer which is preferably applied to the film as an aqueous dispersion. The invention further relates to a process for producing the film and to its use.
An example of a known use of white-colored, biaxially oriented polyester films is as lids for yoghurt cups. They are used there as an alternative to aluminum films. Such lids are printed and embossed on the outer side and provided on the inner side with a varnish or hotmelt, which sticks the lids to the cups. To improve the shelf life of the yoghurt, both the cup and the lid have to ensure adequate light protection. This is generally provided by coloring the lid with suitable pigments. A further requirement on the lid film is that, when it is removed from the cup, it neither delaminates nor starts and continues to tear. The film obtains the properties required for this purpose by the biaxial stretching and by an appropriately large thickness. The mechanical properties of the film (stiffness, puncture resistance, tear propagation resistance) improve greater than proportionally with the thickness of the film. For use as lids for yoghurt, useful polyester film thicknesses have been found to be from 50 to 90 μm. The thickness of the film cannot be as large as desired, since the material costs grow with the thickness and the sealing cycle times decrease, which is undesirable from an economic point of view. There is an observable trend to lower thicknesses.
Lids of Polyester Films Have a Series of Advantages:
The lid has a smooth, very shiny surface which guarantees excellent color reproduction and therefore has a very pleasing visual appearance.
Compared to conventional material, the film has a puncture resistance which is up to 700% higher, which has the consequence of significantly higher product protection.
The lid is easy to open, without fragments of the lid remaining on the cup, as is observed in the case of conventional materials, for example aluminum films. This property in particular is valued by the consumer.
The complete separation of cup and lid is of great advantage for the recyclability of the two materials. The lid film consists of thermoplastic polyester which has excellent recyclability. The product is therefore environmentally friendly. Moreover, the polyester film, as a consequence of its high stiffness and its good gliding ability (in accordance with the requirement profile, the film has low frictional coefficients), has outstanding processability for use as a lid.
In addition, the lid is absolutely free of metal. This guarantees very high reliability in metal detection, which is ever more frequently used in the production of foods. It is therefore possible with the method to determine the proportion of metallic constituents present in the contents with high precision.
Processors use certain criteria for the further processing of the films to produce yoghurt cups. The testing parameters which are customarily used in the production of lid films are the R value and the emax ratio. The R value can be reported as a measure of the orientation and is measured in the middle of the film web directly after production of the film. The emax ratio describes the orientation distribution over the web breadth (known as the “bow”) and is likewise measured in the production of the film, but at discrete intervals over the entire breadth of the film web (cf. under measurement methods). In the case of transparent films (not the white films of the present context), the R values are in the range from 42 to 48 and the emax values in the range from 2.2 to 2.8. Investigations have shown that these values cannot be transferred from transparent films to the present white films. When the above-specified values are observed for white film, this generally leads to delamination of the film. The film delaminates internally, it tears in the thickness direction, the tear destroys the film and the film continues to tear.
The abovementioned properties are not attained in their entirety by the films known from the prior art. For example, EP-A-0 605 130 describes a multilayer film for use as a lid which has at least one opaque layer of crystalline polyester and at least one transparent layer of crystalline polyester. The film is also described by a deformation index which should be ≧2.5%. In addition, the film can be coated with certain substances, which improves the adhesion to printing inks and/or lacquers. Suitable substances which are listed are certain acrylates. There is no information about the R values and the emax ratio of the film. A film reproduced in accordance with EP-A-0 605 130 (example 1, longitudinal stretching temperature approx. 80° C., stretching ratio in the longitudinal stretching 3.3) delaminated, and also exhibited poor processing behavior, since the film curled after punching to give the lids.
From an economic point of view, it is necessary to permanently reduce the costs for the production of the lid film. An important handle for this purpose is the thickness of the film. A low thickness of the film is accompanied directly by a reduction in the material costs and, as a consequence of shorter sealing cycle times, leads to higher fill rates. However, the thickness of the lid cannot be reduced as far as desired, since this leads to processing and handling problems. Lids of polyester film having too low a thickness have a tendency to tear and to delaminate. There is then no longer any product security nor the advantage of 100% separation of lid and cup. The more pigments and the larger the pigments incorporated into the polyester film, the greater the risk of tearing and of delamination of the lid. Pigments, especially when they are relatively coarse, form weak points in the film at which the destruction of the film begins when it is removed from the cup.
In addition, it is advantageous when the film receives a coating which improves the ability of the film to slip, enables better adhesion to printing inks, adhesives (e.g. hotmelts) and sealing varnishes, but moreover does not change the other properties of the film, for example its optical properties. This is an important point, in particular for possible use of its own regrind in the production of the film, said regrind inevitably containing the coating material.
It is therefore an object of the invention to provide a white, biaxially oriented polyester film, in particular for use as a lid, which features improved properties compared to the polyester films established on the market. In particular, the novel polyester films should feature the following combination of properties:                economic producibility, good windability, good processability;        good adhesion of at least one film side to inks, adhesives, sealing varnishes;        very good removal behavior from the cup, no delamination, no commencement nor propagation of tearing;        good optical properties, especially even when its own regrind has been added in the course of film production, and good light protection properties.        
Attempts to provide a conventional transparent film with the desired combination of properties by addition of pigments failed. The film delaminated on removal from the yoghurt cup.